1. Field of the Invention
This invention relates in general to removal of phosphorus or other elements from wastewater, more particularly to removal of phosphorus from wastewater using waste sludge, and most particularly to removal of phosphorus from wastewater using waste sludge in a flow-through contactor.
2. Description of the Related Art
Excess phosphorus in runoff and wastewaters from animal production facilities can result in eutrophication of watersheds with serious consequences for aquatic life and water quality. One of the largest remaining phosphorus sources is agriculture, through overuse of fertilizers and disposal of animal wastes. As an example, like any animal feeding operation, fish hatcheries generate significant amounts of phosphorus. The phosphorus released in the culturing of fish is distributed between solid and dissolved forms. Many hatcheries employ solid-liquid separation technologies to retain solid wastes, but at the present time, no economic options exist for removal of soluble phosphorus from hatchery wastewater. The soluble phosphorus is difficult to capture because of dilute concentrations in high flows of water. However, the cumulative phosphorus load of such operations is contributing to the nutrient loads of flowing waters, leading to degradation of stream water quality.
The problem of excessive soluble phosphorus loading of receiving waters is widespread in many parts of the country. For example, Pennsylvania currently operates thirteen fish hatcheries across the state to support a sport fishing industry that generates millions of dollars of income annually to the region. Recently, the state's Big Springs hatchery near Chambersburg, Pa. was closed because of degradation of the receiving stream through the release of solids and nutrients. As a result, the annual production of over 500,000 trout was lost. Estimated costs of renovation of the hatchery ranged from 2 to 17 million dollars. In addition, several of the state's hatcheries are located on the Susquehanna River or its tributaries. The Susquehanna is the largest river draining into the Chesapeake Bay, and currently contributes about 34% of the total phosphorous load to the bay. Under the Clean Water Act, the EPA has declared portions of the Chesapeake Bay to be impaired because of low dissolved oxygen concentrations resulting from excessive nutrient inputs. The recent Chesapeake 2000 agreement lists steps to be taken to reduce nutrients loads sufficiently so that the impaired designation can be removed. If this is not achieved, a mandatory cleanup will be implemented by the USEPA, requiring a Total Maximum Daily Load (TMDL) for the bay and its tributaries, with high costs of compliance. The steps to be taken include definitions of water quality conditions, followed by assignment of load reductions to each tributary entering the Chesapeake Bay. This will undoubtedly exert pressure on aquaculture facilities to decrease phosphorus emissions significantly. All of these factors are forcing hatchery operators to examine options to decrease phosphorus discharges to the environment. An economical method of prevention of phosphorus release would enable the hatchery system to continue to supply the fish required for the sport fishing industry, while at the same time restoring the hatchery receiving waters to the pristine aquatic environments they once were.
Phosphorus is typically removed from municipal and industrial wastes through the addition of aluminum or iron salts such as alum. These salts precipitate when mixed with neutral waters to form a heavy floc blanket that settles through the water column and removes phosphorus from solution. Phosphorus removal occurs by a combination of mechanisms, including adsorption by the aluminum or iron oxide floc (equations (1) or (2)), and direct precipitation of aluminum or iron phosphate (equations (3) or (4)).Al2(SO4)3.18H2O+3Ca(HCO3)23CaSO4+2Al(OH)3+6CO2+18H2O  (1)2FeCl3.6H2O+3Ca(HCO3)23CaCl2+2Fe(OH)3+6CO2+12H2O  (2)Al+3+PO4−3AlPO4  (3)Fe+3+PO4−3FePO4  (4)
For complete removal of phosphorus, two to three times the stoichiometric requirement is usually needed. However, based on recently published costs for alum and iron salts, the cost of these chemicals generally prevents the use of this technology for the dilute high flows found in aquaculture wastes. Calcium compounds such as lime are sometimes used for phosphorus removal, as shown in equation (5), but require an elevated pH to achieve good phosphorus removal, which is not practical for large flows to be discharged into the environment.5Ca+2+3PO4−3+OH−Ca5(PO4)3(OH)  (5)
Reusable gel-based absorbants have been developed for aquaculture use, but also appear to be too expensive for use in large quantities.
Therefore, it is desired to provide a method for removing phosphorus from aquaculture wastewater streams that is economical.